There is well known a differential gear device including: a differential casing rotatable about an axis and having a pair of through-holes; a pinion shaft accommodated within the differential casing, so as to extend through the through-holes; a pair of pinion gears fitted on an outer circumferential surface of the pinion shaft such that the pinion gears are rotatable relative to the pinion shaft; a pair of side gears meshing with the pinion gears; and a pair of pinion washers interposed between the differential casing and the respective pinion gears. JP-2008-128440A discloses an example of such a differential gear device. In the differential gear device disclosed in this publication, each of the differential casing and the pinion shaft has pin holes, and the pinion shaft extending through the through-holes of the differential casing is fixed to the differential casing, with fixing pins each of which is inserted in the pin holes. JP-63-91746U, JP-1-98952U, JP-2004-68873A, JP-2009-166140A and JP-8-135756A disclose similar prior art differential gear devices of the type as described above.
In the differential gear device wherein the pinion shaft is fixed to the differential casing with the fixing pins inserted in the pin holes formed in the differential casing and the pinion shaft, as disclosed in JP-2008-128440A, the differential casing is required to have a wall thickness large enough to form the pin holes therein. On the other hand, the wall thickness of the differential casing is required to be reduced for reducing the weight and cost of manufacture of the differential gear device. In this respect, there has been a need to improve the prior art differential gear device.